Transistor synchronous rectifier



Dec. 5., 1961 G. M: FORD 3,012,182

TRANSISTOR SYNCHRONOUS RECTIFIER Original Filed Aug. 15, 1957 INVEN TOR. 65 P410 M F0190 ATTQB/VEXS United States Patent Ofiice 3,012,182 TRANSISTOR SYNCHRONOUS RECTIFIER Gerald M. Ford, Santa Monica, Calif., assignor to the United States of America as represented by the Secretary of the Navy Original application Aug. 15, 1957, Ser. No. 678,459, now Patent No. 2,930,984, dated Mar. 29, 1960. Divided and this application May 28, 1959, Ser. No. 816,652

3'Claims. (Cl. 321-47) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This application is a division of my application, Serial No. 678,459, filed August '15, 1957, now United States Patent No. 2,930,984 granted March 29, 1960, and relates generally to demodulators and more specifically to synchronous rectifiers incorporating transistor switching means.

It has been the practice in the art involving choppers and demodul-ators that, in such apparatus, the elements utilized for converting alternating current (A.C.) voltage into a direct current (D.C.) output signal have been electromagnetically actuated mechanical vibrators, or switches, operated synchronously from a source of A.C. reference voltage common to both the chopper and demodulator. Certain disadvantages have been associated with use of these devices. For example, the inertia of the vibrating element of these units is such that any change in operating frequency necessarily introduces a corresponding shift in the phase of the output signal precluding their use in systems where the operating frequency is subject to variation. Moreover, electromagnetically actuated switches also have low reliability factors as the result of mechanical wear and the degeneration of electrical contacts. In the instant invention the aforestated disadvantages of the prior art are avoided through the use of a semiconductor switching element in a synchronous rectifier unit.

In accordance with a typical embodiment of this invention, a synchronous rectifier, incorporating a transistor, is operative in response to an AC. reference voltage for changing an A.C. input voltage into a D.C. output signal. The novel synchronous rectifier of this invention may comprise a semiconductor switch having a bidirectionally conductive path coupled in series with a resistance capacitance network between a source of A.C. signal to be rectified and a ground source of constant reference potential. A source'of A.C. reference voltage is applied to the switch to control the opening and closing of the bidirectional path, and the unidirectional output potential is developed across the network.

The objects of the invention are as follows:

(1) To provide an inertialess, variable frequency, small, light weight, reliable, electronic,-synchronous rectifier; u

(2) To provide an electronic synchronous rectifier requiring only a single'semiconductor as 'a switching element.

It is probable that additional objects and features of the invention will become apparent after reference to the following detailed description made in conjunction with the accompanying drawings wherein:

'FIG. lrepresents an embodiment of ,a novel synchronous rectifier as utilized in the invention,

FIG. 2A represents reference voltage, input voltage, and D.C. output potential fluctuations for the embodi- 3,012,182 Patented Dec. 5, 1 961 l at a time when the waves of the reference voltage E and input voltage E, are one hundred eighty degrees out of phase.

As is well understood in the art, a transistor is a semiconductive body having a region of one conductivity type usually sandwiched between two regions of the opposite conductivity type. A large area metallic connection to the middle region is called the base electrode while similar connections to the end regions are called the emitter and collector electrodes, respectively. A unique characteristic of the transistor which makes it especially useful for switching purposes is its capacity to conduct current in either direction between the two end regions, depending, of course, upon the polarity of the bias potentials applied thereto. The middle region of the transistor, for example, may function as a switch blade. Thus, when a potential having the polarity normally required to interrupt the conductive path between the emitter and collector is applied to the base of the transistor, and the magnitude of this potential is greater than the magnitude of the greatest potential of the same polarity applied either to the emitter or collector, conduction through the transistor ceases, thereby effectively opening the switch. Conversely, whenever the magnitude of the base potential drops below that of a potential of the same polarity applied to the collector or emitter, or whenever a potential of the opposite polarity is applied to the base, the conduction through the transistor is restored, thereby effectively closing the switch.

The embodiment of the novel synchronous rectifier, FIG. 1, comprises a transistor 11 of the P-'N-P type having its collector electrode coupled to a source of A.C. input signal E its emitter coupled through a resistor 13 and the resistance-capacitance network 15 tov a ground source of constant reference potential, and its base electrode coupled to a point common to series connected resistors 17 and 19. The series connected resistors 17 and 19 form a voltage divider between a source of A.C. reference voltage E and a ground source of constant reference potential. The resistance-capacitance network 15 comprises capacitor 21 and resistor 23 coupled in parallel. The D.C. output signal :13, is taken from a common to resistor 13'and the network '15. I

The input and output signal waveforms for two conditions of operation of the synchronous rectifier of FIG. 1 are represented in FIG. 2A and FIG. 2B. As shown in FIG. 2A, a D.C. output signal E of negative polarity is produced when the A.C. input signal E, is in phase with the A.C. reference voltage E,. On the other hand, when the A.C. inputsignal is one hundred eighty degrees out of phase with the A.C. reference voltage E a D.C. output signal E of positive polarity is produced. The waveforms for the latter are represented in FIG. 213. It should be noted that the A.C. input signal E and the A.C. reference voltage E must always be related in phase by an integer multiple of one hundred eightydegrees,

' integer being understood to include zero as well as all ment of FIG. 1 when the waves of the reference voltage whole numbers.

Consider the operation of the synchronous rectifier of FIG. 1, as shown in FIG. 2A when the AC; input signal E is in phase with the A.C. reference voltage 15,. Assume that capacitor 21 is charged to a negative potential having a magnitude slightly less than that of the maximum amplitude of the negative half cycle of A.C. input signal E This potential is represented approximately in FIG. 2A, for explanatory purposes, by the dotted horizontal lines designated 13,. It is an important feature of this invention that the maximum amplitudes of the A.C.

point I to the base electrode of transistorswitch 11 is always biased positive with respect to the potentials present on the emitter and collector electrodes and, as a result, transistor switch 11 is open; currentcannot fiowbetween the emitter and collector electrodes in either direction, and

theD.C. output signal E rernains constant, as shown in the-bottom curve of FIG. 2A.

At time t; the negative half cycles of B and E begin.

At, this instant the negative potential E exists on the emitter'ele'ctrode of transistor switch 11 because of the negative charge stored on the upper plate of capacitor 21. As E and E; become more negative during the interval between t and t the'potential E, on the base electrode becomes negative more rapidly than the potential E on the collector electrode, and the pre-existing potential on the emitter electrode E established by the charge remaining in capacitor 21, is more negative than either E or E As a result, the collectorelectrode is positive relative to the emitterelectrode, and there is an inverse flow of electron current from emitter to collector. Consequently, there is a partial discharge of capacitor 21 through resistor '13.

The discharge of capacitor 21 is represented. by the slight rise in the waveform of the DC. output signal -E shown A .in FIG. 2A between 1 and When, at time t the amplitude of E becomes equal to the magnitude of the pre -existing negative potential 15,, on the emitter, the collector electrode becomes relatively more negative than .the emitter electrode and electron current begins to flow in the normal direction from collector to emitter, thereby again charging capacitor 21 and causing'the magnitude of DC. output signal -E,, to be The voltage relationship of the synchronous rectifier of FIG. 1 whenE and B are out of phase is represented in FIG. 213. .Under this condition of operation, the synchronous rectifier produces a DC. output signal E of positive-polarity. T Assume that capacitor. 21 has 'apositive charge on its upper plate slightly less than the maximum positive amplitude of the .A.C. input signal E and that the maximum amplitudes of the A.C. reference voltage E are greater than those of E The latent positive charge on capacitor 241 establishesa positive potential more becomes relatively negative with respect to the emitter potential E As a result, the switch is closed andelectron current again flows in the normal direction from collector to emitter, thereby causing still further discharge of capacitor 21. Attime t thecollector potential estab-,

lished by the AC. input signal E begins to become more positive than the emitter potential E established by the latent charge on capacitor 21 and the switch, remaining closed on account of the negative swing of thepotem tial on the base electrode established by A.C. reference potential E begins to pass electron current in the inverse direction, from emitter to collector, thereby increasing the positive charge on the upper plate of capacitor 21 and causing the DC output signal +E to become more positive, as shown in the lower waveform of At time 2 the potential. of the emitter and collector electrodes of the transistor switch again reverses, causing electron current to flow in the normal direction and again discharging'capacitor 21 in thernanner described above for the time interval t to t,. Thus, the synchronous rectifier begins a new cycle of operation similar to the i one just described.

Although the transistor. 11 represented in the embodiment of FIG. 1 is ofthe P NP;type, it should beap-. parent that transistors of the NP,N type also maybe used in lieu thereof. .However, the use of a transistor of the N-P-N type will reverse the polarity of the DC. outi put Signah. It shouldbe noted that satisfactory results maybe obtained by reversing the emitter and collector connectionsshown in FIG. 1 notwithstanding the type of transistorutilizedtherein.

The details illustrated'in the accompanying drawings. and set forth in the foregoing description are intended I merelyto facilitate the practice of the invention by persons skilled in. the art. The scope of the invention is rep- 1 rectifier begins andproceeds in the manner just described.

E on the emitter electrode of transistor switch 11, ap-

proximatcd, for explanatory purposes, by the horizontal dotted lines; of FIG. 2B-

During the time interval r to t the AC. input signal E is negative, going, t-hereby establishinga potential on the collector which is relatively more negative than the potential E on the emitter of transistor 11. -As a result, electroncurrent flows in the normal direction from collector to emitter and partially discharges the positive po- 'tential on the Upp r plate of capacitor 21, an effect representedin the bottomwaveform of FIG. 2B. This disresented in the following claims.

What is claimed is:

1. A synchronous rectifier for converting an alternating current input signal in-to'a direct current output signal having a polarity,representativeof the phase relationship between, said. input signal and an alternating current reference voltage and having a magnitude representative of the amplitude of said input signal, said synchronous rectifier comprising:v a transistor having collector, emitter,

and base 'electr odes means coupling the bidirectional collector-to-ernitter pathof the said transistor in 86116511218:

tion with. the combination consisting oi firstresi stance. coupled in series with a capacitance and a second resist-.

ance coupledin parallel with said capacitance between a source of alternating current input lsignala nd a ground source of referencefpotential; means for applying an alternating current reference voltage having an amplitude i greater than thatiof saidinput signal, having a frequency equal to that of said input signal, and being-related in phase'to said input s'ignalfbiy anintegral multiple I ofone-hundredeighty degreestothe said base electrode such/that said traznsistor is rendered conductive and nonconductiveon alternate halt cycles of said input signnal;

and output means coupled to a common point between saidfirstandsecond resistances for supplying during each chargingcurrent continues until, at time n, the" positive f potential established on the base electrode of the transis-' tor-11by the positive going swing of AC. reference voltage becomes equal to theernitter potential E thereby I causing 'thecessation' of electron ,current fiow by theopeningof switch 111. f

During .the timeflinterval' t to t the positive swing of AC. referencevoltage E maintains transistor switch 1 1 in open positionand the D.C. output, signal -+E,, re-

. mains constant as shown in theflower waveform, of'iFIG,

.At tiinefln, E putts negative going, downswing once successive half-cycle 'a continnous... direct current, output signal from saidsync-hronous rectifier.

2. A synchronous rectifier fonconverting an alternating current input signal intoa direct current output signal 'having a polarity. indicatiye. of the phase relationship betweensaid input signal and an alternating current refw er'ence voltage and having a -magnitudelindicative ofthe amplitude of said input signal, 'said' synchronousrectifier comprising:- a transistor having abidirectionallyconductivejpathfcoupled in series withthe combination consistingot a, first. resistance coupled in series with a..capacitance and a second resistance coupled, inparallel with said capacitance between a sourceof alternating,currentvolt 5 ageand .ag'r und sourceotc t nt; refer n epotential i lin a voltage divider made up of first and second series connected resistors coupled between a source of alternating current reference voltage having an amplitude greater than that of said input signal, having a frequency equal to that of said input signal, and being related in phase to said input signal by an integral multiple of one hundred eighty degrees, and the said ground source; means coupling a point common to the first and second resistors to a current controlling electrode of the transistor; and an output terminal means coupled to a common point between said first and second resistances for supplying during each successive half-cycle a continuous direct current output signal from said synchronous rectifier.

3. A synchronous rectifier for converting an alternating current input signal into a direct current output signal having a polarity indicative of the phase rela tionship between said input signal and an alternating current reference voltage and having a magnitude indicative of the amplitude of said input signal, said synchronous rectifier comprising: a transistor switch means having first and second conduction electrodes for providing a bidirectional conduction path, and a control electrode; a first terminal means for receiving an alternating current input signal coupled to said first conduction electrode; a second terminal means for receiving an alternating current reference voltage having an amplitude greater than that of said input signal, having a frequency equal to that of said input signal, and being related in phase to said input signal by an integral multiple of one hundred eighty degrees, coupled through a first resistance to said control electrode of said switch means, said control electrode being further coupled through a second resistance to ground potential whereby said first and second resistances provide a voltage dividing arrangement between said second terminal means and said control electrode; the combination consisting of a third resistance having one terminal coupled to said second conduction electrode and the other coupled through a storage means to ground potential, and a fourth resistance coupled in parallel with said storage means whereby an output means coupled across said fourth resistance supplies a continuous direct current output signal during each successive halt-cycle of operation of said synchronous rectifier.

References Qited in the file of this patent UNITED STATES PATENTS 2,884,545 Houck Apr. 28, 1959 2,898,476 Jensen Aug. 4, 1959 2,953,738 Bright Sept. 20, 1960 OTHER REFERENCES A Transistor D.C. Chopper Amplifier by P. L. Burton, published in Electronic Engineering (August 1957), pages 393-397 relied on. 

